Around 40 per cent of children and one in three adults around the world are regularly exposed to secondhand smoke, a substance that kills more than 600,000 people every year, World Health Organization researchers say.
Those deaths are on top of the 5.1 million deaths that smoking itself causes every year.
The study, which appears in The Lancet medical journal, is the first to look at the global impact of secondhand smoking, and focused on 192 countries -- some with restrictions on smoking, and some with none at all.
Scientists concluded that "passive" smoking causes about 379,000 deaths from heart disease, 165,000 deaths from lower respiratory disease, 36,900 deaths from asthma and 21,400 deaths from lung cancer a year. Altogether, they account for about one per cent of the world's deaths.
"Two-thirds of these deaths occur in Africa and south Asia," the researchers, led by Armando Peruga, a program manager at the WHO's Tobacco-Free Initiative, wrote in their study.
The authors write that many of the children exposed to secondhand smoke also have to contend with infectious diseases, a combination they called "deadly."
They noted that kids whose parents smoke have a higher risk of sudden infant death syndrome, pneumonia, bronchitis and asthma. Their lungs may also grow more slowly than kids whose parents don't smoke.
Commenting on the findings in an accompanying editorial, Heather Wipfli and Jonathan Samet from the University of Southern California said families should be motivated to stop smoking in the home.
"In some countries, smoke-free homes are becoming the norm, but far from universally," they write.
To reach their findings, the WHO researchers looked at data dating back to 2004 and then used mathematical modeling to estimate deaths and the number of years lost of life in good health.
They found that worldwide, 40 per cent of children, 33 per cent of non-smoking men and 35 per cent non-smoking women were exposed to second-hand smoke in 2004.
The researchers also found that while there were more child deaths due to secondhand smoking in poor and middle-income countries, deaths in adults were spread across countries at all income levels.
Secondhand smoke appears to have its biggest impact on women, killing about 281,000. In many parts of the world, women are at least 50 per cent more likely to be exposed to secondhand smoke than men.
The researchers also noted that only 7.4 per cent of the world population currently lives in areas with smoke-free laws, yet even in those areas, laws are not always strongly enforced.
But in places where smoke-free rules are adhered to, exposure to secondhand smoke in high-risk places, such as bars and restaurants, can be cut by 90 per cent, and in general by 60 per cent, the researchers said.
